The operation of conventional clutch arrangements for heavy duty vehicles requires a relatively high level of operator skill in order to provide for smooth starts and shifts, particularly when the vehicle is heavily loaded. This expertise is required for the driver to appropriately correlate accelerator position and the rate at which the clutch is engaged when changing the clutch from a fully disengaged to a fully engaged position. If the engine speed is too high and the clutch is engaged suddenly, there is a tendency for the truck to rear or buck before the entire vehicle attains proper forward motion. If on the other hand the engine speed is too low to provide sufficient power to move the vehicle, then the engine may stall as a result of the clutch engagement. In addition to these operational characteristics, failure to smoothly and properly complete the engagement of the clutch in relation to the engine speed and amount of engine acceleration can result in excessive loading factors on the various elements in the drive train with resultant shortened life characteristics of these elements. Additional adverse effects may arise from allowing the clutch to engage too slowly and to slip such that excessive heat is generated in the clutch causing thermal damage in the clutch elements themselves.
In order to eliminate the degree of skill required by the driver and to obtain long life in the drive train and clutch components, various techniques have been employed to produce automatically controlled clutches. With automatically controlled clutches, not only may the above referred to problems be overcome, but, additionally, the physical requirements for the driver become easier and the entire shifting operation can more nearly approximate that of an automatic transmission in a passenger car. One approach to automatic clutch engagement, which has not proved suitable for heavy duty automotive clutches, is one in which there is not completed positive coupling, that is, where the drive shaft is not ever completely mechanically linked to the driven shaft, but rather various degrees of coupling are permitted and by controlling the degree of coupling in accordance with engine speed, a controlled relationship between the speed of the driven shaft and the drive shaft may be achieved. However, such clutches, which usually employ electromagnetic coupling, are inefficient inasmuch as there is never a complete positive linkage and additionally, such couplings usually generate considerable heat from the lost power in the non-complete coupling.
Another approach has involved the use of a clutch control mechanism in which the clutch is moved from a completely disengaged position through an intermediate controlled series of positions to a final engagement in which there is a positive mechanical linkage. Such a system is described in U.S. Pat. No. 2,144,074 issued Jan. 17, 1939. In this system, in a first stage after initiation of re-engagement, the clutch members are moved slowly toward engagement until a specific engine speed is reached and, at this engine speed a second stage is initiated in which the clutch engagement is controlled in accordance with engine speed in a predetermined relation. In the final stage the clutch engagement is completed. In this system, however, there is no relationship established between throttle position and the amount of clutch engagement, or between the acceleration of the engine and clutch engagement. Thus, the relationship between the engine speed and clutch engagement is a fixed one, irrespective of the throttle position. Additionally in this technique, there is no provision for limiting the heat dissipation in the intermediate stage to prevent excessive thermal damage to the clutch components.